The invention relates to a method and a device for forming a socket on a pipe of thermoplastic material. In particular the invention relates to a method comprising the following steps:
an end section, to be provided with a socket, of the pipe is heated in a heating station,
the pipe with the heated end section is transferred to a socket-forming station which comprises a socket-forming mandrel, the socket-forming station being set up in such that a pipe can therein be provided with a socket, while the end section of another pipe can simultaneously be heated in the heating station,
the socket-forming mandrel is introduced into the pipe end section, heated to a suitable socket-forming temperature, and the end section is formed into a socket,
the pipe is cooled in the region of the socket, and
the socket-forming mandrel is removed from the pipe.
In the context of the production of plastic pipes having a socket it is generally known to make use of a production installation which comprises a heating station and a socket-forming station positioned next to it. An example of this is described in U.S. Pat. No. 4,113,813.
In the case of that known installation, the pipe end section which is to be provided with a socket is first heated in the heating station, so that the end section becomes soft and readily deformable. Then the pipe with that soft end section is transferred to the adjacent socket-forming station, where the socket-forming mandrel is introduced into the soft end section and said end section is formed into the desired shape of the socket. After cooling of the socket the pipe is ready for use.
In WO 97/33739, the present applicant described a production installation which is used for forming a socket on a pipe of biaxially oriented thermoplastic material. This known installation does not have separate heating and socket-forming stations, but instead has a single station in which both the heating of the end section and the forming of the socket take place. To this end, the headmost end of the socket-forming mandrel is designed as a projecting cylindrical section which forms a socket-forming mandrel support section that fits into the pipe. In the process, the pipe is pushed onto the support section and then the end section of the pipe is heated to a suitable socket-forming temperature. Next, the socket-forming mandrel is pushed further into the pipe, so that the heated end section of the pipe slides over the socket-forming section of the socket-forming mandrel.
A drawback of the production installation of the type as shown in U.S. Pat. No. 4,113,813 is that the warm and soft end section of the pipe, when the pipe is transferred to the socket-forming station, can readily become damaged or deformed, for example indented. It should be noted in this context that even damage that at first sight appears to be of little significance can give rise to a non-uniform wall thickness or strength of the formed socket.
In the case of the production installation according to WO 97/33739, no transfer of the pipe having a heated end section to a socket-forming station takes place, so that the abovementioned drawback thus appears to have been overcome. A disadvantage, however, is that the sockets which have been formed using said installation have inadequate quality and that the production capacity is lower than would be desirable. As regards quality, it was found that in a large number of cases cracking occurred in the sockets. Sometimes these cracks formed as early as during production, but also if the pipe was subsequently subjected to a (impact) load.
It is an object of the present invention to provide a method and a device by means of which pipes of thermoplastic material, in particular of biaxially oriented thermoplastic material, can be provided with sockets of a high quality. It is also an object of the present invention to provide a method and apparatus having a high production capacity.
The present invention provides a method wherein prior to the pipe end section being heated in the heating station, a support is introduced into the end section of the pipe, said support extending substantially over the length of the end section in order to internally support the end section. According to this method the support remains in the end section, when the pipe with the heated end section is transferred to the socket-forming station, after which the socket-forming mandrel is introduced into the pipe and in the process the support is pushed deeper into the pipe.
According to a preferred embodiment the end face of the end section is thermally shielded. This measure is based on the insight that in the region of the end face of the pipe, the ratio between the external pipe surface area, which without said shielding would be exposed to the heating effect of the external heating means, and the volume of the pipe wall is greater than in that section of the pipe which is located further away from the end face. Without the shielding, more heat per unit mass of pipe material would then be supplied in the end section region than further on. This would produce a distinctly higher temperature at the outermost edge of the end section, directly next to and in the end face, than in the adjoining pipe wall sections. One of the factors involved is the poor heat conduction of the plastic. Furthermore, the end face is always found to have a rough surface, in particular as a result of the operation by means of which the pipe has been shortened to the desired length. In conjunction with the locally elevated temperature, said roughness leads to hairline cracks being formed in the region of the end face of the pipe. When the socket is formed, these hairline cracks grow into considerable cracks. The thermal shielding prevents said overheating and consequently the risk of cracking.
The present invention also provides a device for forming a socket on a pipe of thermoplastic material.
Further advantageous embodiments of the method and device according to the invention are described in the claims and in the following description with reference to the drawings.